Modulation



MODULATION Filed octfzz, 1931 #www Img- Bi `/1 27 L. USSELLMAN G BY/INVENTOR ATTORNEY sTATEs PATENT oFFlcE:

MODULATION George L. Usselman, Port Jefferson, N. Y., as-

srgnor to Radio Corporation of America, a corporation of DelawareApplication October 22, 1931, Serial No. 570,343 1 claim. (c1. 17e- 171)This invention relates to signalling and more in second tube will resultin frequency modulation of particular to signalling by oscillations thefrethe oscillator. In practice, there may be some quency of which isaltered in accordance with the amplitude modulation of the oscillationsgeneramplitude of the signals. ated in the oscillator. This effect istaken care of 5 More in detail, this invention relates to a new by theamplitude limiting device connected with 5 and improved means ofproducing high frequency the output of the oscillation generator.oscillations by means of an oscillation generator In the modification,the output of the oscillaand for producing signal oscillations whichdetertion generatoris inductively coupled to the amplimine the frequencyof the oscillations generated tude limitor. In both modifications signalfreby the high frequency generator. quencies applied to the inputcircuit of the second 10 In one form of my invention this is accomtubemay be amplified or not as necessity requires. plished by providing athermionic oscillator hav- The circuit arrangement and the operation ofing coupled input and output circuits. The outmy invention in itsbroadest aspect is as set forth put circuit of this generator isconductively couabove. For .purposes of detailed illustration,

l5 pled through a capacity to an amplitude limitor specific embodimentsof my invention will be set l5 so that the amplitude of the oscillationsproduced out more indetail hereinafter.

by the generator may be held at a constant Value In setting forth morein detail the present inbythe amplitude limiter. Signal modulations arevention, reference will be made to the attached impressed on the inputcircuit of a second therdrawing throughout which like referencecharmionic tube. The output circuit of the last named acters indicatelike parts, and in which: 20 tube includes an inductance which isinductively Figure 1 shows one embodiment of the invencoupled to theinductances in the input and output tion; while f circuit of thethermionic oscillator generator. n Figure 2 shows a modification of thearrange- The oscillator is biased to a point such that conment in Figure1.

tinuous oscillations are produced. The normal Referring 'moreparticularly to the drawing, l 25 bias applied to the grid of the secondtube is such indicates a thermionic oscillation generatorhavthatacurrent of predetermined intensity will flow ing an anode 2, anda control electrode 3 conin the plate circuit thereof, A part of thiscurnected through an inductance 4 and a Variable rent will be rectifiedcurrent resulting from curcapacity 5Min a well known manner to producerent induced from the oscillator circuit windings sustainedoscillations. The inductance 4, shunted 30 IltO the iIlduCaIlCe in 611Plaie CYCU 0f the Secbya variable condenser 6, forms the-tank circuitond tube. The oscillator will then generate oscilfor the oscillationrgenerator 1. The frequency lations at frequencies determined by theamount of the oscillations normally generated by 1 is deof currenttransferred from the oscillator intermined by the inductance 4 andvariable capacductance to the inductance in the plate circuit of ity 6,The Circuits between the anode 2 and 35 the second tube. When modulatedSignals are cathode Fand grid 3 and cathode F are completed applied t0the input CilCUlit 0f the SSCOnd tube, by blocking condenser 8 andground as shown.

the characteristics thereof will vary in accordance The necessarybiasing potential is applied to the with the nature of the IIlOdulaingSignals. The control electrode 3 by means of a battery 10 concurrent inthe plate circuit of the second tube will nected to the deem-0de 3through a Choking in 40 COIlSeqlleIli/ly Vary in BJCCOTdanCe With theBaillie ductance 12. Direct current potential is sup- Of the modulatingSignal The resistance 0f the plied to the anode circuit of theoscillation genpla'e CathUdB Circuit Will Change- This Will re' eratorby a conductor 1l, the source 9 to the centersult in correspondingchanges in the amount of tap on inductance mtiaton of the urrent inducedfrom the osc' The oscillator as shown is for purposes of illuslait-101.1lnductances mto the mductance m the plat tration since, obviously, anyoscillator known to-v circult of. the second time' These .Chatlges resulday can be used. For example, the tuned grid, directly in a change inthe effective inductance t t d l t o -11 t W-th tht of the oscillatorcircuits, and will cause a change n ed or un une p a e ou@ a or' l ,1SWe of frequency in the oscillations generated by the gul'he modulator.1S assoclated Wlth the 50 oscillator. The frequency generated by theoscillation generator will vary substantially inversely The modulatortub? 20 Includes a? anode 21 as the mductance of the Oscillator Circuitsconnected through an inductance 22 withasource of high potential 23vshunted by a bi-pass conchanges. In this manner any signal amplitude nmodulation impressed on the input circuit of the denser 30. Inductance22 1s inductively coupled 55 as indicated with the inductance 4. Themodulating frequencies from a source 24 are impressed through atransformer 25 on the input circuit of tube 20. Biasing potential issupplied to the control electrode 26 from a source 27 connected inseries between the control electrode and the filament 29. The condenser30 prevents any audio or high frequency oscillations induced frominductance 4 into inductance 22 from reaching the direct currentpotential source. The nmodulation source may include amplifiers. ThefrequencyY and amplitude of-.the modulating oscillations produced in 24may be of any value.

The tankcircuit Yof thermionic oscillator generator 1 isrconducti'velycoupled through blocking'condenser 32 to an amplitude limiting device34. This amplitude limiting device Yforms no part of the presentinvention andneed not be described in detail herein. The amplitudelimiting device should be such that the amplitude of the modulatedsignals fed thereto ,from the tank circuit may be maintained constantover the frequency range covered by the generator.

, The operation is as follows: The oscillator generator 1 and themodulator tube 2G are energized. The oscillation generatori is producingoscillations ata frequency predetermined by the elements in the tankcircuit 64. No modulating energy is being impressed on the input circuitof the modulator 20. The biasing potential applied to the controlelectrode 26 of modulator 20 by source 27 is such, however, that currentof predeter- Wmined intensity flows between the anodeY cathodeYimpedance of 20. The tube 20 under these cond.tionsY is conductive to acertain extent. Oscillations are being produced by the generator 1 andare induced from the inductance 4 in the tank circuit thereof to theinductance22 in the anode cathode circuit of modulator 20. Theseoscillations induced in 22 will cause rectified current to flow in theanode cathode circuit of tube 20. Under these conditions, the effectiveinductance of 4 and 22 will be such that a certain frequency will begenerated by the oscillation generator 1. Now assume that modulatingfrequencies of varying amplitude are impressed on the input circuitoftuber20. These amplitude modulations flowing in the input circuitthereofwwill vary the potential applied normally-to the controlelectrode 26. This will result in variations at signal frequency in theimpedance of tube 2U. The intensity of the current flowing in the anodecathode circuit including the inductance 22 of tube 20 wili vary inaccordance with the above changes. Since part of this current representsthe current induced from 4 to 22 which has been rectified, the amount ofcurrents which may be induced from 4 to 22 will vary also in accordancewith the variations in signal frequency. This results in correspondingchanges in the effective inductance of the inductance 4 in the tankcircuit of 1. The frequency generated by l depends in part on theinductance of the output circuit thereof and said frequency will varysubstantially inversely as the inductance in 4 varies. Therefore, whensignal amplitude modulation is applied from 24 to the input circuit of20, frequency modulation of the oscillator l will result. The tankcircuit is conductively connected through a blocking condenser 32 to theamplitude limiting device 34 in order that in the event some amplitudemodulation of the frequency generated by 1 results, it may be taken careof or eliminated to the degree necessary by the amplitude limitor 34.The output of the limiting device 34 is fed to an amplifier and/orfrequency multiplier 35 and from 35 to a radiating system A or otherload circuit.

The modification shown in Figure 2 is substantially the same as themodification shown in Figure 1. In Figure 2, however, the tank circuitinductance 4 is inductively coupled to the Yinductance 36 in the inputcircuit of the limiting device 34. Furthermore, in this modification,the source of signal modulations, indicated as microphone 36, isconnected through the transformer 37 to a modulation frequency amplier38 having its output circuit ccnnected'to the primary winding of thetransformer 25. l

Although for purposes of illustration IV-have set VVforth in detail thearrangement and operation of several modifications of my invention, itwill be understood that I do not wish to be limited thereby except asmarked out in the claim appended hereto. Y

Having thus described my invention and the operation thereof, what Iclaim is:

In a system for producing frequency Ymodulated energy, the combinationof Yan oscillator tube having an anode, a cathode, and a controlelectrode, an impedance and a source of potential connected in seriesbetween said control electrode and cathode for polarizing said controlelectrode to a suitable operating potental, a parallel tuned circuitcomprising an inductance coil and a variablecondenser connected inparallel, a connection between one Yterminal of said parallel tunedcircuit and said anode, a variable condenser connecting the otherterminal of said parallel tuned circuit to said control electrode, aby-passinggcondenser connecting an intermediate-.point of saidinductance coil to said cathode, means for energizing the anode of saidtube, whereby oscillations are set up by said tube at a frequencydependent upon the tuning of said parallel tuned circuit, and, a circuitfor varying the frequency 'of said parallel tuned circuit, saidfrequency varying circuit comprising a second vacuum tube having aplate, a grid, and a ilament, a coil connected between said plate andlament, said coil being magnetically coupled to said coil of saidparallel tuned circuit, a low frequency circuit interconnecting the gridand filament of said last named tube, and, a source of low frequencycurrents coupled to said last mentioned circuit whereby variations inthe current flowing through said last mentioned vacuum tube varies byvirtue of the varying current iiowing through said coil connectedbetween the plate and cathode of said last named tube, the inductance ofsaid parallel tuned circuit and hence the frequency of the oscillationsproduced by said oscillator tube.

GEORGE L. USSELMAN.

